Considerable research and development work has been done to develop convenient techniques for operating on transmitted information-carrying light to produce various device functions. Devices known variously as hybrid optical integrated circuits, photonics modules and hybrid optical packages have been proposed for controlling light using planar waveguides that transmit light in a manner similar to the transmission of electricity by the electronic circuit patterns of printed circuits and integrated circuits.
The paper, "Glass Waveguides on Silicon for Hybrid Optical Packaging," C. H. Henry et al., Journal of Lightwave Technology, Vol. 7, No. 10, October 1989, pp. 1530-1539, and the patent of Blonder et al., U.S. Pat. No. 5,182,787, granted Jan. 26, 1993, are examples of the literature describing methods for making and using planar optical waveguides. Such methods generally use successive layers of glass over a silicon substrate to define optical waveguides. One layer of glass having a relatively high refractive index is known as the core layer and is surrounded by glass known as the clad layers having a lower index of refraction. During operation, light is confined in the core glass because of the lower refractive index of the surrounding clad layers and, as a consequence, the configuration of the core layer defines the path of the light. Photolithographic masking and etching can be used to define the waveguide paths with precision, and the Henry et al. paper describes how various optical devices such as couplers and polarization splitters can be fabricated in this manner.
The use of a planar optical waveguide implies that light is introduced into one end of the waveguide and extracted from the other end. Efficient coupling of light to and from a waveguide normally requires a lens, and for this purpose properly located small spherical lenses of glass, ball lenses, are often used. If the separate fabrication, alignment and packaging of glass lenses could be avoided, planar optical waveguides would be easier and less expensive to make and use.